1. Field of the Invention
This invention relates to electromagnetic ballasts for gaseous discharge lamps and particularly to such ballasts for sequentially starting and simultaneously operating a plurality of gaseous discharge lamps, such as fluorescent lamps.
2. Description of Related Art
U.S. Pat. No. 2,682,014, which is hereby incorporated by reference, describes several electromagnetic ballasts for starting and operating first and second gaseous discharge lamps.
Generally, each ballast includes a transformer having a primary winding P, a first secondary winding S1, and a second secondary winding S2. The windings are serially connected, with the secondary windings arranged in voltage bucking relationship. The first lamp is connected in series with a first capacitor across the series combination of the primary winding P and the first secondary winding S1. The second lamp is connected across the series combination of the first and second secondary windings. A second capacitor is also connected in series with the second lamp and the second secondary winding.
In operation, when the primary winding is energized by an AC supply voltage, both the primary winding P and the first secondary winding S1 will produce combined voltages which will be sufficient to ignite the first lamp. As a result, current will flow through the first secondary winding S1. Because of a high leakage reactance of winding S1, it will produce a voltage in phase with and additive to a voltage induced in the second secondary winding S2. These combined voltages will ignite the second lamp. With both of the lamps operating, there is a series path for the major portion of the current through the lamps, the first and second capacitors and the second secondary winding. The first secondary winding S1 is effectively bypassed because of its high leakage reactance, which impedes the flow of current through it. Such a first secondary winding is typically known in the art as a start winding or, alternatively, as a tickler winding. Because it carries so little current after both lamps have ignited, the tickler winding S1 typically comprises a large number of turns of very fine wire.
The function of the second capacitor is to protect the tickler winding against damage in the event that the second-to-start lamp L2 begins to function as a rectifying tube. This sometimes happens after long hours of operation of this lamp and results from the loss of emission material from one of the lamp electrodes. In that case, but for blocking action of capacitor C2, a pulsed DC current would flow through the lamp L2 and potentially damage or destroy the tickler winding.
Although the two-capacitor type of ballast described in U.S. Pat. No. 2,682,014 was effective in protecting the tickler winding from failure, it produced an unacceptable difference in the current and power delivered to the two lamps. It also produced starting currents which were too low to reliably ignite energy saver lamps.
U.S. Pat. No. 4,740,731, which is hereby incorporated by reference, describes a two-capacitor ballast having improvements for overcoming the above-mentioned problems. Schematically, the configurations of the ballast embodiments disclosed are similar or identical to those disclosed in U.S. Pat. No. 2,682,014. They also operate in generally the same way. However, in order to improve operating characteristics the capacitance of the second capacitor was limited to the range C1xe2x89xa6C2xe2x89xa61.5(C1). Further, a slot in a portion of the transformer magnetic core structure around which the second secondary winding is wound had a transverse dimension (width) in the range of 25-50% of the width of the respective core portion. Preferably, the slot had a width approximately 35% of the core portion. A slot width of 65% was found to be unsatisfactory. Further, the ratio of the number of turns of the first secondary winding to the second secondary winding was approximately 1.53.
While the two-capacitor ballast described in U.S. Pat. No. 4,740,731 might have solved the current imbalance and starting reliability problems of the earlier ballast, it was not a commercial success. In particular, it produced unacceptably high vibration-noise levels and used too much power to comply with later-enacted Federal legislation setting minimum efficiency standards. It also passed an undesirably high AC current through the first secondary winding (tickler winding) when the second lamp was non-functional (i.e., inoperative or missing).
It is an object of the invention to provide an electromagnetic ballast of the above-described type which overcomes all of the above-mentioned problems.
It is another object of the invention to provide such a ballast which has a substantially-higher energy efficiency rating than comparable electromagnetic ballasts.
In order to achieve the above and other objects, a design study was undertaken which began with analyzing the operation of the basic single-capacitor ballast upon which U.S. Pat. No. 2,682,014 sought to improve. New design criteria were established which not only aimed at avoiding the above-mentioned problems, but also taking advantage of the lighting efficiencies of lamps currently on the market. In essence, the inventor reinvented the two-capacitor electromagnetic ballast. Although it schematically resembles ballast embodiments disclosed in U.S. Pat. Nos. 2,682,014 and 4,740,731, and operates in a similar manner, its design parameters are quite different.
In accordance with the invention, an electromagnetic ballast for sequentially starting and simultaneously operating first and second gaseous discharge lamps comprises a transformer including a magnetic core, a primary winding for connection to an AC voltage source and first and second secondary windings, all wound around the core. The first and second secondary windings are wound in opposite directions to produce voltages in opposition to each other. The ballast includes first and second series circuits. The first series circuit includes the primary winding, the first secondary winding, a first capacitor and the first lamp. The second series circuit is electrically connected in parallel with the first secondary winding and includes the second lamp, the second secondary winding and a second capacitor. The magnetic core has an elongated slot formed under the second secondary winding. The core and the slot have respective widths, substantially in a direction transverse to lines of flux produced in the core, such that a ratio of said slot width to said core width lies in the range of 60 to 70%. In a preferred form of the invention, the slot has a length in a direction substantially parallel to the lines of flux produced in the core, which slot width is much larger than the slot length.